1. Field of the Invention
The present invention relates to piezoelectric resonators utilizing extensional vibration. The present invention also relates to piezoelectric components and ladder filters for surface-mounting. Further, the present invention relates to ladder filters having piezoelectric resonators utilizing extensional vibration.
2. Description of the Related Art
The structure of a conventional piezoelectric resonator 1 utilizing extensional vibration is shown in FIG. 1. In the piezoelectric resonator 1, a piezoelectric plate 2 is polished to have a thickness T and electrode films 3a and 3b are provided on the principal planes of the piezoelectric plate 2 such that the piezoelectric plate 2 is polarized in the thickness direction by polarization processing. When T is the thickness of the piezoelectric resonator 1, and L1 and L2 are lengths of edges thereof, the capacitance across terminals (capacitance between electrodes) Cf is defined by the following equation (1),
Cf=(xcex5oxc2x7xcex5sxc2x7L1xc2x7L2)/Txe2x80x83xe2x80x83(1),
wherein: xcex5o=dielectric constant under vacuum; and xcex5s=specific dielectric constant of the piezoelectric plate.
The resonance frequency fr of the extensional vibration of the piezoelectric resonator 1 is defined by the following equation (2) under the condition L1≅L2,
fr=V/L1xe2x80x83xe2x80x83(2),
wherein V=wave motion speed propagating in the piezoelectric plate 2, and V≅2000 m/sec. Therefore ,when the required resonance frequency fr is determined, lengths of edges L1=L2 of the piezoelectric resonator 1 are determined according to the equation (2).
Accordingly, in order to increase the capacitance across terminals Cf of the piezoelectric resonator 1 at a predetermined resonance frequency fr, a piezoelectric material having a large specific dielectric constant xcex5s needs to be selected or the thickness T of the piezoelectric plate 2 needs to be reduced according to the above equation (1).
However, when the specific dielectric constant xcex5s of the piezoelectric material used for the piezoelectric plate 2 is increased, other piezoelectric constants such as a piezoelectric quality constant Qm and an electromechanical constant k are also changed, so that only the capacitance across terminals Cf cannot be increased without changing other piezoelectric constants. On the other hand, when the thickness T of the piezoelectric plate 2 is excessively reduced, the piezoelectric resonator 1 is liable to crack when subjected to external impacts caused for example by dropping. Thus, this method of reducing the thickness of the piezoelectric plate 2 has limitations. When L1≅L2≅4.5 mm (fr=450 kHz), the thickness T at the limit of the strength required is empirically and approximately 300 xcexcm.
Piezoelectric components used for surface-mount ladder filters are disclosed in Japanese Unexamined Patent Publication No. 8-18382 and Japanese Unexamined Patent Publication No. 7-176977. In these piezoelectric components, a plurality of piezoelectric resonators and metallic terminals which are aligned in a line alternating in the vertical direction are accommodated in a case and external electrodes are provided by bending lead portions of the metallic terminals so as to extend outside of the case.
However, these piezoelectric components have a large external thickness, so that the piezoelectric component protrudes greatly from a surface of a substrate when being mounted on the substrate, thereby causing difficulties for use as a circuit substrate in a low-profile device and preventing a device from being modified into a low profile device.
In these piezoelectric components, the required number of metallic terminals is increased. For example, when a component includes four piezoelectric resonators, four to five metallic terminals are required. Thereby, the cost of materials is increased and the assembly time is increased, resulting in added costs necessary to produce the piezoelectric component.
The circuit diagram of a conventional four-element type (two-stage) ladder filter 201 is shown in FIG. 14. The ladder filter 201 has two series-connected resonators 204S and 205S connected together in series between an input-terminal 202 and an output-terminal 203 and two parallel-connected resonators 206P and 207P respectively inserted between each of the output-sides of the respective resonators 204S and 205S and the ground. When the ladder filter is assembled to produce an actual component, the combination of two series-connected resonators and two parallel-connected resonators are vertically sandwiched together via a terminal board, and is accommodated into a case. (For example, Japanese Unexamined Utility Model Publication No. 4-76724).
The guaranteed attenuation ATTo of such a ladder filter is defined by the following equation (3), when the capacitance across terminals of the series-connected resonators 204S and 205S is Cfso and the capacitance across terminals of the parallel-connected resonators 206P and 207P is Cfpo,
ATTo2xc3x9720Log(Cfso/Cfpo)xe2x80x83xe2x80x83(3).
Referring to FIG. 15, in the conventional series-connected resonators 204S and 205S and the parallel-connected resonators 206P and 207P utilizing extensional vibration, on both surfaces of a piezoelectric plate 208 having a square shape electrodes 209 are provided. When Ls is the edge length of the series-connected resonators 204S and 205S, Ts is the thickness thereof, xcex5s is the specific dielectric constant thereof, and xcex5o is the dielectric constant under vacuum, the capacitance Cfso across terminals of the series-connected resonators 204S and 205S is defined by the following equation (4),
Cfo=(xcex5oxc2x7xcex5sxc2x7Ls2)/Tsxe2x80x83xe2x80x83(4).
Likewise, when Lp is the edge length of the parallel-connected resonators 206p and 207p, Tp is the thickness thereof, xcex5p is the specific dielectric constant thereof, and xcex5c is the dielectric constant under vacuum, the capacitance Cfpo across terminals of the parallel-connected resonators 206p and 207p is defined by the following equation (5),
Cfpo=(xcex5oxc2x7xcex5pxc2x7Lp2)/Tpxe2x80x83xe2x80x83(5).
Therefore, substituting the values of the equations (4) and (5) into the above equation (3) yields the following equation (6) for obtaining the guaranteed attenuation ATTo,
ATTo=2xc3x9720Log [(xcex5sxc2x7Ls2xc2x7Tp)/(xcex5pxc2x7Lp2xc2x7Ts)]xe2x80x83xe2x80x83(6).
Furthermore, the resonance frequency of the series-connected resonators 204S and 205S and the resonance frequency of the parallel-connected resonators 206p and 207p are both determined by their respective sizes (edge lengths Ls and Lp). When a ladder filter having the required frequency is manufactured, the sizes of the series-connected resonators 204S and 205S and the parallel-connected resonators 206p and 207p are the same (Ls=Lp), so that the guaranteed attenuation ATTo( less than 0) is defined by the following equation (7),
ATTo=2xc3x9720Log [(xcex5sxc2x7Tp)/(xcex5pxc2x7Ts)]xe2x80x83xe2x80x83(7).
Therefore, in order to increase the guaranteed attenuation ATTo (the absolute value), it is necessary that the specific dielectric constant xcex5s of the series-connected resonators 204S and 205S is reduced while the specific dielectric constant xcex5p of the parallel-connected resonators 206p and 207p is increased. It is also necessary that the thickness Ts of the series-connected resonators 204S and 205S is increased while the thickness Tp of the parallel-connected resonators 206p and 207p is decreased.
However, when considering other piezoelectric characteristics such as a piezoelectric quality constant Qm, an electromechanical constant k, and temperature characteristics, it is preferable that the same piezoelectric material be used for the series-connected resonators 204S and 205S and the parallel-connected resonators 206p and 207p. It is difficult to select piezoelectric materials for the series-connected resonators 204S and 205S and the parallel-connected resonators 206p and 207p individually relative to only the specific dielectric constants xcex5s and xcex5p.
Also, with decreasing the thickness Tp of the parallel-connected resonators 206p and 207p, the strength thereof is reduced so that the element itself is susceptible to break (cracking or other damage), while with increasing the thickness Ts of the series-connected resonators 204S and 205S, the ladder filter itself will be heavier and much larger. Due to the above-mentioned limitations in manufacturing, the conventional ladder filter has another drawback in that the guaranteed attenuation ATTo cannot be freely designed.
To overcome the problems discussed above, preferred embodiments of the present invention provide a piezoelectric resonator which is arranged such that capacitance across terminals have a large value without decreasing the strength thereof and without having different piezoelectric materials being used for the piezoelectric plates.
Further, preferred embodiments of the present invention also provide a piezoelectric component including a plurality of piezoelectric elements or a ladder filter having a plurality of series-connected resonators and parallel-connected resonators that have a low-profile and which are suitable for surface mounting.
Further, preferred embodiments of the present invention provide a ladder filter having a large amount of guaranteed attenuation while having series-connected resonators and parallel-connected resonators that have approximately the same thickness and are made of the same piezoelectric material.
In a preferred embodiment of the present invention, a piezoelectric resonator vibrating in an extensional vibration mode includes a plurality of piezoelectric plates, and a plurality of electrode films. The piezoelectric plates and the piezoelectric films are alternately disposed so as to be integrated and so as to define a monolithic piezoelectric body. Also, a first set of the plurality of electrode films are electrically connected to each other while a second set of plurality of electrode films are electrically connected to each other.
With these features of the piezoelectric resonator, a piezoelectric element vibrating in an extensional mode includes a piezoelectric plate and electrode films provided on both major surfaces of the piezoelectric plate and the piezoelectric resonator has a plurality of these resonator elements, thereby enabling the capacitance across terminals to be increased without changing the size of the piezoelectric plates and while using the same piezoelectric materials for the resonator elements.
In addition, since the piezoelectric resonator has a structure in which plurality piezoelectric plates are arranged to define an integral, unitary member, even when the capacitance across terminals is increased by reducing the thickness of the piezoelectric plates, cracking of the piezoelectric plates is prevented. Therefore, the capacitance across terminals of the piezoelectric resonator is significantly increased without decreasing the strength of the piezoelectric resonator although the thickness of each piezoelectric plate is reduced.
Preferably, a piezoelectric resonator also includes a plurality of side surface electrodes preferably disposed at nodal points on external end surfaces of the monolithic body of the plurality of piezoelectric plates and the plurality of electrode films, wherein some of the plurality of electrode films are electrically connected to each other by some of the plurality of side surface electrodes while the rest of the plurality of electrode films may be electrically connected to each other by some of the other plurality of side surface electrodes.
With these features of the piezoelectric resonator, since a side surface electrode is provided substantially at a nodal point on the external surface of the monolithic body, the extensional vibration of the piezoelectric resonator is not affected or damped by the arrangement of the side surface electrode.
In another preferred embodiment of the present invention, a piezoelectric device includes a plurality of piezoelectric plates, and a case, wherein the plurality of piezoelectric plates are arranged in one plane of the case.
With these features of the piezoelectric device (including a ladder filter), a plurality of plate-shaped piezoelectric elements are arranged in one plane in the case, thereby achieving a low-profile piezoelectric device. Therefore, when being mounted on a circuit board or other similar electronic devices, the mounting height is not increased, thereby contributing to low-profile apparatuses. The structure of the metallic terminal can also be simplified by arranging the piezoelectric elements in one plane, so that the number of required metallic terminals is also greatly reduced.
In the piezoelectric device, the plurality of piezoelectric plates preferably have substantially the same thickness.
With these features of the piezoelectric device, because of the piezoelectric elements having substantially the same thickness, no space is wasted in the case, thereby contributing to a low-profile piezoelectric device. Since the thickness of each piezoelectric element is substantially the same, assembly of the device is also improved when assembling with terminals.
In another preferred embodiment of the present invention, a ladder filter includes series-connected plate-shaped resonators, parallel-connected plate-shaped resonators having internal electrodes disposed in the thickness direction thereof, and a case. The series-connected plate-shaped resonators and the parallel-connected plate-shaped resonators are arranged in one plane of the case.
With these features of the ladder filter, since the series-connected plate-shaped resonators and the parallel-connected plate-shaped resonators are arranged in one plane in the case, a low-profile ladder filter is achieved. Therefore, when being mounted on a circuit board or other electronic devices, the mounting height is not increased thereby contributing to low-profile apparatuses. The structure of the metallic terminals are simplified by arranging the ladder filters in one plane, so that the number of required metallic terminals is reduced.
Furthermore, since the parallel-connected resonators have a deposited structure of electrodes in the thickness direction, the capacitance across terminals of the parallel-connected resonators is significantly increased while using the same piezoelectric materials and while reducing the thickness of the parallel-connected resonators, which results in increasing the guaranteed attenuation of the ladder filter.
In another preferred embodiment of the present invention, a ladder filter includes series-connected resonators, parallel-connected resonators including a plurality of piezoelectric plates and plurality of electrodes. The parallel-connected resonators have the electrodes disposed in a thickness direction thereof and such that the piezoelectric plates are interposed between the electrodes. A case is also provided, wherein the series-connected resonators are stacked on each other on one side of the case and the parallel-connected resonators are stacked on each other at another side of the case.
With these features of the ladder filter, since the parallel-connected resonators have a structure in which electrodes are deposited in the thickness direction by having piezoelectric plates interposed between the electrodes, the distance between the electrodes is reduced without reducing the thickness of the parallel-connected resonators, resulting in increased capacitance across terminals of the parallel-connected resonators. Accordingly, the guaranteed attenuation of the ladder filter is also increased. In particular, when forming the series-connected resonators and the parallel-connected resonators with the same material, the capacitance across terminals of the parallel-connected resonators is increased larger than that of the series-connected resonators and the guaranteed attenuation of the ladder filter is preferably increased.
Preferably, the ladder filter further includes an external electrode disposed substantially at a nodal point of vibration of the parallel-connected resonators, wherein the electrodes are connected together by the external electrode.
With these features of the ladder filter, the electrodes are connected together by the external electrode substantially located at a nodal point of vibration of the parallel-connected resonators, so that the capacitance across terminals of the parallel-connected resonators is increased by parallel-connecting each electrostatic capacitance across the electrodes. Moreover, since the external electrode is provided substantially at a nodal point of vibration, piezoelectric vibration of the parallel-connected resonators is not restrained.
In the ladder filter, the thickness of the series-connected resonators is preferably substantially the same as the thickness of the parallel-connected resonators.
With these features of the ladder filter, when the thickness of the series-connected resonators and the parallel-connected resonators are substantially the same, the capacitance across terminals of the parallel-connected resonators is increased larger than that of the series-connected resonators, so that the guaranteed attenuation of the ladder filter is increased. When the thickness of the series-connected resonators and the parallel-connected resonators is substantially the same, it is not necessary as in the conventional ladder filter that the thickness of the parallel-connected resonators be reduced to increase the guaranteed attenuation, thus preventing cracking of the resonator. On the other hand, if the thickness of the series-connected resonators is increased, this results in a heavy or large ladder filter. Further, by providing a resonator with series-connected resonators and parallel-connected resonators having substantially the same thickness, simplification of the assembly process is also achieved.
Other features, elements, and advantages of the present invention will be described in detail below with reference to preferred embodiments of the present invention and the attached drawings.